Ceramics have a good thermal conductivity and an insulation property, and they are good packaging materials. Generally in the usage, the surface metallization of ceramics is required for fabricating electronic circuits or filler metal electronic components. In addition, as the result of the brittleness and the difficulty for machining, a ceramic needs to be attached with a metal to make composite materials or components in many cases.
Traditional processes of surface metallization comprise a noble metal process and a Mo—Mn process. The noble metal process first mix noble metals powder, such as silver powder or palladium powder with glass powder, binder and solvent to form a slurry and then to coat it on a surface of a ceramic, and then to sinter it on the surface of the ceramic to form a noble metal film by heating to a temperature about 900° C. to melted the glass. The Mo—Mn method is to mix molybdenum powder and manganese oxide powder to make a slurry and to coat it on a surface of a ceramic, and then to sinter a molybdenum metal on the ceramic by forming a combined oxide of manganese and the ceramic at a temperature of about 1500° C. The former uses expensive raw materials, and the bonding strength is lower, therefore, is generally used in making electronic conducting film; the latter has a higher bonding strength between the metal film and the ceramic, because that molybdenum has a thermal expansion coefficient near that of ceramics and therefore has a lower thermal stress after sintering. Thus besides the usage for making electronic conducting film, the molybdenum metal film on the surface can also be used for the brazing of ceramics with metal, by using the commonly used brazing alloy.
In addition to the Mo—Mn metallization/brazing process, the commonly used processes for bonding ceramics with metals also comprise a DBC process and an active-metal-brazing process. The DBC process first laminates an oxygen-containing copper plate or a surface-oxidized copper plate on a ceramic plate and then heat to about 1070° C. in an inert atmosphere to form a Cu—Cu2O eutectic melt on the surface of copper, to attach the ceramic with the copper plates by using the melt as brazing filler. The active-metal-brazing process first mixes silver, copper, and titanium powder to make slurry and then coats on the ceramic surface before laminating copper or special steel or other metal materials/components. Then the ceramic and metal materials or components are brazed together by heating to about 850° C. in a vacuum atmosphere, to have the silver-copper-titanium filler melted. The DBC process is mainly used for making composite circuit boards of ceramics and copper (Due to the large difference of thermal expansion coefficient between ceramics and copper, and the thermal stress developed in the composite, which can be large enough to cause the fracture of ceramics, the thickness and shape of copper are strictly restricted). The active-metal-brazing process is more widely used in the manufacturing of ceramic-metal composite components, owing to that a varieties of metals can be brazed, and the thermal stress can be decreased by brazing a metal having a thermal expansion coefficient near that of ceramics: such as composite circuit boards of ceramics and copper, automotive turbocharger rollers and engine tappets. The Mo—Mn process is also widely used in the manufacturing of the ceramic cases of silicon rectifiers and the bonding of metal electrodes.
Aluminum is a kind of metal materials whose in-use amount is only second to steel. Owing to the chemical activity of aluminum itself, aluminum can react directly with ceramics to bond on ceramics theoretically. Furthermore, as the yield strength of aluminum is relatively low, the thermal stress in the ceramics-metal composite can be relieved by plastic deformation. Therefore, the bonding of aluminum and ceramic has a broad prospect. However, due to the impediment of primary oxide film on the surface of aluminum, the bonding of ceramics and aluminum is difficult.
At present, many researches on the bonding of ceramics and aluminum have been executed, and a lot of bonding processes have been developed. Such as a vacuum brazing process, a solid phase diffusion bonding process, a friction welding process, a high-vacuum cleaning/pressing process and an ultrasonic vibration bonding process. The vacuum brazing process is to place aluminum alloy filler metal with a low melting point between a ceramic and aluminum, and then to heat in a nitrogen or other inert gases atmosphere, or in a high vacuum atmosphere with a vacuum higher than 10-3 Pa to melt the filler metal so as to bond the ceramics and aluminum together. The solid phase diffusion bonding process is basically the same as the vacuum brazing process. The difference is that no filler metal is used and the bonding is conducted at a temperature below the melting point of aluminum. Since no liquid is produced in the bonding process, a large enough pressure is necessary so that the surfaces of aluminum and a ceramic can be contacted with each other. Although aluminum and ceramics can be bonded by using these processes, but owing to that the chemical property of aluminum is extremely active, and its equilibrium partial pressure at a temperature lower than 1.000° C. is less than 10-40 Pa, the oxidation of aluminum cannot be avoided even in the presently available maximum vacuum conditions. An amorphous aluminum oxide film always exists at the bonding interface of a ceramic and aluminum. As a result, a large number of defects are formed at the bonding interface, leading to a rather bad or a fluctuated mechanical property of the bond, which in turn hinders the application of these processes. [X. S. Ning. T. Okamoto, Y. Miyamoto, A. Koreeda. K. Suganuma, and S. Goda, Bond strength and interfacial structure of silicon nitride joints brazed with aluminum-silicon and aluminum-magnesium alloys. Journal of Materials Science. Volume 26 (1991) 2050-2054; E. Saiz; A. P. Tomsia; K. Sugamuma, Wetting and strength issues at Al/α-alumina interfaces, Journal of European Ceramic Society. Volume 23 (2003) 2787-2796].
The friction pressing or the ultrasonic vibration pressing, or the ultra-high vacuum cleaning/pressing processes are developed for eliminating the effects of oxide film. The friction pressing process is to remove the surface oxide film of aluminum by mutual grinding of the surfaces of a ceramic and an aluminum part under pressure, and then use the heat generated by the friction to press aluminum and the ceramic to bond with each other. The basic principle of ultrasonic vibration pressing is just the same as friction pressing. The difference is that the friction is generated by the ultrasonic vibration in the ultrasonic vibration pressing process. Ultra-high vacuum cleaning/pressing process is first to removes the oxide film on the surface of aluminum by ion bombardment in a vacuum environment, and then to presses aluminum and ceramics together in 10-6 Pa ultra-high vacuum environment to bond them. Although these processes can more or less remove the oxide film on the surface of aluminum and can improve the performance of the bonding interface of aluminum and ceramics, a large pressure need to be applied during the bonding processes, which results in the distortion of aluminum part. In addition, the shape of the metal or ceramic parts is strictly limited.
If an aluminum film can be formed on the surface of a ceramic, the bonding of a ceramic to aluminum will be transformed to the bonding of aluminum themselves. Thus, the well-developed techniques for the brazing of aluminum can be used to the bonding of the ceramics and aluminum. However, the formation of aluminum film on the surface of ceramics is rather a difficult problem. Efforts have been made to form an aluminum film on the surface of alumina by using a process of vacuum evaporation, and a magnetron sputtering or a molecular beam epitaxy process, but it is found that a continuous aluminum film cannot be formed on the surface of alumina. Furthermore, as the temperature of the alumina substrate is kept at a temperature near room temperature, the aluminum vapor deposits rapidly on the alumina substrate, but deposits to many isolate islands; but if the temperature of the substrates exceeds the melting point of aluminum, no aluminum can be deposited on alumina. [G. Dehm, B. J. Inkson. T. Wagner. Growth and micro-structural stability of epitaxial Al films on (0001) α-Al2O3 substrates. Acta Mater., volume 50 (2002)5021-32; M. Vermeersch, F. Malengreau. R. Sporken. R. Caudano, The aluminium/sapphire interface formation at high temperature: an AES and LEED study, Surf. Sci., Volume 323 (1995) 175-187.] This is because that the wettability of aluminum with ceramics is relatively poor, (currently most of the measured wetting angles are larger than 75°). According to the theory, a substance whose wetting angle to a substrate is larger than zero actually cannot form a continuous film on the substrate. By the way, the unbond-defects generally form at the brazing interface of ceramics and aluminum can also be related to the fact that aluminum cannot spread over the surface of ceramics.